Does Javascript event queue have priority?

Question

These days, I have read some documents about setTimeout and setInterval. I have learned that the Javascript is a single thread which will only execute one piece of code per time. At the same time, if there is a event happens, it will be pushed into the event queue and block until appropriate time. I want to know, when many events are blocked waiting to execute at the same time. Do these events have different priorities, so the high priority event will execute before the low ones. Or just a FIFO queue.

setTimeout(fn1, 10);
$(document).click(fn2); //will be called at 6ms;
$.ajax({ajaxSuccess(fn3); //async request,it uses 7ms;})

 for () {
    //will run 18ms;
};

In the above code, the setTimeout fn1 will happen at 10 ms,click event handler fn2 will at 6ms, ajax callback fn3 will at 7ms, but all the three functions will be blocked until the for loop finish. At 18ms, the for loop finished, so what order does these functions will be invoked.(fn1,fn2,fn3) or (fn2,fn3,fn1)

Answer 1

Does javascript event queue have priority?

Sort of. The event loop is actually composed of one or more event queues. In each queue, events are handled in a FIFO order.

It's up to the browser to decide how many queues to have and what form of prioritisation to give them. There's no Javascript interface to individual event queues or to send events to a particular queue.

https://www.w3.org/TR/2014/REC-html5-20141028/webappapis.html#event-loops

Each task is defined as coming from a specific task source. All the tasks from one particular task source and destined to a particular event loop (e.g. the callbacks generated by timers of a Document, the events fired for mouse movements over that Document, the tasks queued for the parser of that Document) must always be added to the same task queue, but tasks from different task sources may be placed in different task queues.

For example, a user agent could have one task queue for mouse and key events (the user interaction task source), and another for everything else. The user agent could then give keyboard and mouse events preference over other tasks three quarters of the time, keeping the interface responsive but not starving other task queues, and never processing events from any one task source out of order.